1. Field of the Invention
The invention relates to an imaging system and method in which an imaging device is used to provide an electrostatic charge image in accordance with the varying amount of incident light or x-ray energy received by the device, the system and method providing for the conversion of the electrostatic charge image into electrical signals usable for producing the image in a visible form.
2. Description of the Prior Art
Radiation sensitive devices referred to as metal insulator semiconductor type (MIS) or metal oxide semiconductor (MOS) devices are disclosed in U.S. Pat. Nos. 3,497,698 and 3,746,867 to Robert J. Phelan, Jr. et al. These devices require a substantial charge accumulation (depletion) region in an n-type (p-type) semiconductor layer adjacent the dielectric layer. Imaging radiation absorbed in this narrow accumulation (depletion) region produces charge carriers which must be capable of being transferred into the dielectric and, as a result, transform the charge accumulation (depletion) region into a charge depletion (accumulation) region. Electronic detection of the change in the nature of this narrow region of the semiconductor adjacent to the dielectric is accomplished by scanning a radiation beam across this interface and detecting a resulting photo-voltaic electrical signal that is indicative of the original imaging radiation. This device has limited utility since practical devices are small (a few cm..sup.2 at most) and preferably operated at very low temperatures. Also, since only charges photo-generated in the narrow charge accumulation region are transferred into the dielectric, the device is relatively insensitive to highly penetrating imaging radiation such as x-rays.
A radiation sensitive device in the form of a conductor-insulator-semiconductor (CIS) structure is used as the storage element of the device disclosed in U.S. Pat. No. 3,916,268 to William E. Engeler. This storage element is provided with an initial charge which is then modified by the generation of minority carriers in the semiconductor in response to application of radiation to the device. The change in the charge is a measure of the integrated radiation energy. Readout of the charge then present is made by electronically interrogating the device causing the device to discharge providing an electrical signal indicative of the charge that was present. A measure of radiation that has been applied to various portions of an area can only be obtained by using a large number of the devices within such area which can be formed in an integrated array on a substrate. The devices of the array can be sequentially addressed and discharged subsequent to a charging time-interval to obtain an electric readout of the incident radiation each device received.
U.S. Pat. No. 3,970,844 to John B. Fenn, Jr. et al discloses a system in which an electrostatic charge image is found at the surface of a photoconductive layer in accordance with the x-ray energy absorbed by an ion emitting medium, such as gas, located between the x-ray source and the photoconductive layer. An electrode is positioned between the ion emitting medium and the x-ray source. While the x-ray energy is presented, an imaging power supply is connected between the electrode and an optically transparent conductive layer carried by the surface of the photoconductive layer away from the x-ray source causing the electrostatic charge image to be formed at the surface of the photoconductive layer. The imaging power supply is then disconnected. Readout electronics are connected to the conductive layer for receiving signals corresponding to the magnitude of charge at various points on the photoconductive layer in response to the scanning of the photoconductive layer by a light source operated under the control of the readout electronics. Several different scanning methods are disclosed. The system requires that the photoconductive layer be non-x-ray absorbing or that a layer of x-ray absorbing material be positioned at the surface of the photoconductive layer adjacent the ion emitting medium, the material being electrically anisotropic so the charge image is transferred to the photoconductive layer.